Recent instabilities with regard to price and availability in the world oil market have provided an incentive for energy users to seek alternative fuel sources. One such alternative energy source results from the gasification of a solid carbonaceous fuel, such as coal, wood chips, oil shale, heavy oil, etc., for the production of a clean fuel gas which may be used for a wide variety of industrial and utility processes.
The gasification of a carbonaceous fuel is a relatively simple process, involving the reaction of carbon with water vapor to form carbon monoxide and hydrogen gas. As this reaction is endothermic, it is usually necessary to release at least a portion of the chemical energy of the solid fuel as heat energy by adding an excess amount of oxygen to the reactants. The excess oxygen allows combustion of at least a portion of the solid fuel within the gasification reactor thus releasing energy to drive the remaining gasification reactions. The resulting products of combustion, CO.sub.2 and H.sub.2 O, dilute the produced fuel gas, thus reducing its heating value. If excess oxygen for the combustion reactions is supplied by using air, a quantity of inert nitrogen is also present within the produced fuel gas, reducing the heating value even further.
When certain fuels, such as coal, heavy oil, and waste hydrocarbons, are heated within a closed container a portion of the fuel is driven off as a gas. This process, know as devolatilization, results in the production of a hydrocarbon gas with a relatively high heating value, about 2670 to 4450 kcal/m.sup.3 (300 to 500 BTU/cubic ft.). For carbonaceous fuels, the solid remaining after devolatilization is primarily carbon and any other inert compounds which may be present. This coke or char may be further gasified under the proper conditions of temperature and pressure by the addition of water vapor thus allowing the carbon-water gasification reaction to take place. The heat energy to drive the devolatilization and gasification reactions may be provided indirectly by means of heat transfer surface in contact with the solid fuel. Indirect gasification systems, although known in the prior art, have generally proved to be unsatisfactory due to their inability to effectively transfer heat from the heat source to the fuel and due to their inability to completely gasify all of the carbon in the remaining fuel solids.
Another drawback with prior art indirect processes results when sulfur is present within the feed fuel, as is often true with carbonaceous fuels such as coal. The sulfur reacts with available hydrogen to form hydrogen sulfide, H.sub.2 S, a noxious gas which, when combusted, forms sulfur dioxide, an atmospheric pollutant. Gasification systems in the prior art all remove the hydrogen sulfide produced in the gasification reaction by means of downstream scrubbing systems such as the Stretford or Selexol processes, both of which are complex and expensive undertakings.
A large number of industrial processes and equipment as well as certain large scale utility applications require a clean, sulfur-free fuel such as natural gas or highly refined oil. These processes could be easily adapted to use a clean fuel gas with a heating value in the range of 2670 to 4450 kcal/m.sup.3 (300 to 500 BTU/ft.sup.3). What is required is a simple, effective gasification system which produces a clean, relatively high energy content gas from an unsuitable carbonaceous fuel without the need for oxygen producing equipment and without leaving significant quantities of unreacted carbon. The clean fuel gas thus obtained, particularly that which results from the devolatilization process, is useful as a chemical feedstock for the production of certain plastics and other chemical products. The system should also be able to handle a wide variety of solid or liquid fuels and should efficiently utilize all of the heat energy available.